Apparatus and method for diminishing electric fields within containers of flammable material

ABSTRACT

An array of X-ray sources is mounted on the upper surface of a container of flammable fluids. Each X-ray source includes a high voltage generator, an electron accelerating tube and an X-ray target. Each X-ray source is also housed in a metallic enclosure which maintains the source in a water and gas tight manner. The X-rays which are formed as wide angle beams produce free ions within the gaseous region within the container, and these ions neutralize any undesirable electrostatic fields which form within the gas-filled regions inside the container.

United States'Patent [191 Trump APPARATUS AND METHOD FOR DIMINISHING ELECTRIC FIELDS WITHIN CONTAINERS OF FLAMMABLE MATERIAL [75] Inventor: John G. Trump, Winchester, Mass.

[73] Assignee: High Voltage Engineering Corporation, Burlington, Mass.

[22] Filed: Sept. 7, 1973 [21] Appl. No.: 395,042

[52] US. Cl 250/494, 250/493, 250/492 [51] Int. Cl. H0lj 37/00 [58] Field of Search 250/492, 493, 494

[56] References Cited UNITED STATES PATENTS 4/19'30 Henocque et al 250/494 2,463,569 3/1949 Smith 45] Jan. 21, 1975 Primary' ExaminerJames W. Lawrence Assistant Examiner-B. C. Anderson Attorney, Agent, or Firm-Russel & Nields [57] v ABSTRACT An array of X-ray sources is mounted on the upper surface of a container of flammable fluids. Each X-ray source includes a high voltage generator, an electron accelerating tube and an X-ray target. Each X-ray source is also housed in a metallic enclosure which maintains the source in a water and gas tight manner. The X-rays which are formed as wide angle beams produce free ions within the gaseous region within the container, and these ions neutralize any undesirable electrostatic fields which form within the gas-filled regions inside the container.

17 Claims, 5 DrawingFigures Nau-Touron 250/492 BACKGROUND OF THE INVENTION The present invention relates to an apparatus and a method for diminishing .to spark-free levels the intensity of undesirable electrostatically-generated electric fields within the gaseous spaces of large containers of flammable liquids and gases.

Flammable products in both liquid and gaseous form are stored in large storage containers and transported over waterways in large tankers. One crucial safety concern is the formation of electrostatically induced electric fields withinthe container. If these fields reach the intensity which causes sparking, and if the gaseous regions contain oxygen, the flammable products could ignite or detonate. v

The separation of electric charges and the consequent buildup of potentially dangerous electric fields takes place whenever quantities of matter are in motion, particularly in rapid or violent motion. This is due fundamentally to the fact that all forms of matter exhibit differences in the vigor with which they encourage or resist the detachment of electrons by physical or electrical forces. Thus when two forms'of matter are in frictional contact, one tends to acquire electrons, thereby becoming negatively charged, and the other tends to lose electrons and become positively charged. The transfer of large quantities of insulating hydrocarthrough valves and filters provides many opportunities for such charge separation. The subsequent accumula- I tion in a large tank of an insulating fluid which has had this frictional experience results in high electric fields extending throughout the fluid and gas-occupied regions of the, metal enclosed system. Even though the charge density (coulombs/ cubic meter) is small, the cumulative effect of this net charge in a large system can easily produce voltages of many'megavolts. The effect of these high potentials may be intensified by distor- 'tions in the tank geometry which result in small localized regions of high electric stress approaching the corona or sparkover values. Such a distortion in an otherwise low field situation could be caused by a projecting steel beam or brace or by a grounded conductive rod or wire.

The surface motion in a rolling sea of a large fluidfilled system can likewise cause charge separation and the attainment of dangerous sparking conditions.

In still another process of electrification, the washing down of vapor'filled tanks with high velocity water streams, whether salt water or fresh, can cause local electrification leading to electrical discharges. These discharges would be found particularly in the region of the impact of the water streams on the tank walls. The electrification results from the attendant fragmentation and rupture of the water stream and particles and from the filling of the surrounding space with a cloud of charged water particles. Many other fundamentally similar opportunities for the production of hazardous electric fields occur in systems involving moving insulating fluids or high velocity streams.

The region surrounding isolated electric charge of a given polarity is filled with an electric field due to that charge. This electrically stressed region extends until the field lines can terminate on an equal number of chargesof the opposite polarity. This means that in closed system the field will fill the entire volume extending to everysurface and corner thereof. The distribution (intensity) of electric stress of the field is affected by the geometry of the container including the relative distance of the walls from the isolated charge and by projections from the walls which would serve to intensify the field.

In a closed system containing fluid at the bottom covered by a gaseous vapor phase, the explosion hazard is greatest in the vapor-filled region. Both regions are ocmospheric pressure.

,bon'fluids through pipes and between tanks and SUMMARY OF THE INVENTION According to the present invention, there is provided an apparatus and a method for overcoming the problem previously mentioned by regulating the intensity of the electrostatic field so that it never reaches the sparking level within the gaseous regions of the system: A plurality of modular X ray sources are affixed in a distributed array to the upper surface of acontainer hous ing a vaporizable liquid and its vapor. Each source includes a high voltage generator, an electron accelerating tube and a grounded anode X-ray target. Electric power is supplied to the sources to maintain them in operation during extended periods when electrostatic hazards exist. The X-rays emerge from said sources into the interior of said container as wide angle beams to produce free ions of both polarities throughout the 7 portion of the container housing the vapor.

When X-rays pass through a gaseous medium, their absorption results in ionization of some of these gas molecules. Electrons'are ejected from these molecules,

therebyleaving residual positive ions and electrons.

These electrons attach themselves to neutral molecules. Thus, the result of X-ray absorption is the formation of positiveand negative ions. These ions are acted upon by the electric field that may be present and move in such directions and in such amounts as to substantially neutralize the electrical field in the gaseous region of the system. In the presence of an electric field, the positive ion moves in the one direction and the negative ion in the other; thus the illumination of the gasfilled space by X-rays makes available a large number of charge carriers which are naturally forced by the field to move in directions which will diminish and effectively neutralize this field. Should the volume of the liquid in such a closed system be negatively charged, positive ions would be driven to the surface of the liquid and produce a charge layer upon this surface sufficient to cancel out the field effects which this negative charge had previously created in the gas-filled region.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: FIG. 1 is a schematic diagram of a modular X-ray source in accordance with the present invention;

FIG. 2 is a diagram depicting an array of X-ray sources in a liquid fuel tank;

FIG. 3 is a diagram in perspective of a storage tank having X-ray sources according to the present inventron;

FIG. 4 is a diagram depicting the electric field arising from electrical charges in the storage tank of FIG. 3;

FIG. 5 is a diagram depicting the reduction in the electric field of FIG. 4 by the X-ray source according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS In an exemplary embodiment of the electric field diminishing device according to the present invention, as shown in FIG. 1, a modular X-ray source, represented generally by the reference numeral 10, is mounted on the upper surface of a container 12 of the flammable fluids. Typically, this container 12 is made of steel, and when the container is a tanker, such as in FIG. 2, it may be about one inch thick. The source is enclosed within a dome-shaped upper cover 14 which is suitably secured to a lower plate 16. By this arrangement, the components of the X-ray source 10 are maintained in a water and gas tight manner.

The components of the X-ray source 10 include a high voltage generator 24, an electron accelerating tube 26 and an X-ray target 28. The target 28 extends into an X-ray permeable aperture in the container 12. Preferably, the X-rays are produced by electrons with energies less than 300 kiloelectron volts. A source of electrical power (not shown) operates the high voltage generator 24. In operation, the X-ray source 10 develops wide angle beams of X-rays which radiate into the region within the container as shown at 30 in FIG. 1

and 32 in FIG. 3. I

Preferably, the energy of the X-rays and the X-ray stopping power of the upper container and of the modular sources are designed so that above one meter from the upper surface of the container the X-ray intensity is below the internationally accepted safe limit for human exposure. Also, the X-ray target is maintained no higher than 60C above the temperature of the container 12; this is accomplished by thermal conduction of the buildup of heat in the target through metal to the container 12. The resultant average density of free ions of both polarities within the gaseous space is at least two orders of magnitude higher than the average free ion density produced therein by naturally available ionizing processes.

Referring now to FIG. 2, there is shown an array of X-ray sources 10 for a liquid fuel tanker 32. The lines indicate the various liquid compartments within the fuel tanker 32. The tanker might be full, partly full, or virtually empty. There can in all cases be electrically dangerous fields.

FIG. 3 shows a large storage tank 12 and an array of deck-mounted X-ray sources 10 which irradiate the gaseous region below. Wide angel beams 32 of X-rays are produced so that the X-rays are absorbed by as large a volume of the gas within the tank 12 as possible.

The operation of the present invention is now described with particular reference to the diagrams comprising FIGS. 4 and 5, which show a typical storage tank, such as in FIG. 3. The container 12, which is grounded, is partially filled with a flammable liquid 50. The region above the liquid 50, which is represented generally by the reference numeral 52, contains some oxygen and vapors from the fluid 50. As previously described, friction produced by relative motion between the fluid 50 and other forms of matter, such the walls of the container 50 and fluid transfer pipes. creates an accumulation of electrical charges in the fluid 50. While in FIGS. 4 and 5 the fluid has a net accumulation of negative charges, which means the fluid has an overabundance of electrons, it is also conceivable that the fluid could also have a net positive charge. In FIG. 4, the negative charge is distributed throughout the volume of the fluid 50.

It is important to note that the liquids with which the invention is concerned, namely flammable liquids such as oil, are insulating liquids. An insulating liquid is one that resists the flow of electric charge. This means that charge which may have been frictionally supplied to the volume of the liquid will tend to stay trapped therein for long periods of time. These times could be of the order of minutes, hours and days. In general. by insulating liquid we mean one which has a resistivity higher than 10 ohms/cm. A good insulating oil may have a resistivity of 10 and an excellent insulating oil of 10 ohms/cm. It would take the latter fluid nearly a year to conduct to the surface of the metal container within which it is placed. Since the liquid resists the flow of electrical charge, the electrons cannot readily migrate to the grounded metal container whereby they would otherwise be neutralized. Thus, other means must be provided for neutralizing the effect of the net charge in the liquid.

The region surrounding an isolated electric charge of a given polarity is filled with an electric field due to that charge. This field is represented in FIG. 4 by an array of electric field lines 54 which extend from one side of the container, through the gaseous region 52 and the fluid 50, to the other side of the container. For the reasons previously stated, the danger created by this field is greatest in the gas-filled region.

Referring now to FIG. 5, the effect. of the introduction' of X-rays is to substantially neutralize the electric field in the gaseous part of the system. The X-rays produce ions of both polarities in the gaseous space within the container.

When X-rays pass through a gaseous medium, their absorption results in ionization of some of these gas molecules. This means that electrons are ejected from the molecules leaving a residual positive ion and an electron. No charges are added; the charges are merely separated. The electron in general does not exist as such very long, usually for a very small fraction of a second. There is a strong tendency for electrons to attach themselves to neutral molecules. Thus the end result is that the absorption'of X-ray energy results in the formation of positive and negative ions. In the presence of an electric field, the positive ion moves in the one direction and the negative ion in the other; thus the illumination of the gas-filled space by X-rays makes available a large number of charge carriers which will naturally be forced by the field to move in directions which will diminish and effectively neutralize this field. Should the volume of the liquid in such a closed system be negatively charged as in FIG. 5, positive ions would be driven to the surface of the liquid and produce a charge layer upon this surface sufficient to reduce to spark free levels the field effects which this negative charge had previously created in the gas-filled region above. A corresponding number of negative ions mitime irradiation with X-rays of hydrocarbon fluids at' the intensity levels needed for our purposes.

The present invention comprehends the formation of ions in the gaseous region of a container housing a flammable liquid which has an electrical charge, the effect of the ions being to neutralize the electric field in the gaseous region created by the charge. Although the foregoing description specifically relates to the protection of liquid fuel carrying tankers, the same method is applicable to still other systems in which the hazardous electric field may be produced by the injection of matter in the form of airborne particles into a large container or in transfer through a large conduit. Typical examples are the transfer and storage of combustible materials such as coal and coal dust, and the conveying of flour through a mill.

1 claim:

1. Apparatus for diminishing to spark-free levels the intensity of unwanted electric fields within the gaseous spaces of large containers of flammable liquids and gases including: at least one container housing a vaporizable liquid subject to accumulation of unwanted charge of one polarity and its vapor, a plurality of mod- X-rays emerging from said sources into theinterior of said container as wide angle beams to produce free ions of both polarities throughout said gaseous spaces, those of said ions which have a polarity opposite to that of said unwanted charge moving toward said unwanted charge to neutralize the field effects of said unwanted charge.

' 2. The apparatus according to claim 1 wherein said 7 X-rays are produced by electrons with energies generally less than 300 kiloelectron-volts.

3. The apparatus according to claim 1 further including water-tight and gas-tight metallic enclosures for the X-ray sources. v

4. The apparatus according to claim 3 wherein the energy of said X-rays and the X-ray stopping power of the upper container surface and of the modular sources being such that the level of X-ray intensity 1 meter above this surface is always below the internationally accepted safe limit for human exposure.

5. The apparatus according to claim 1 wherein the target of said X-ray source is maintained at no more than 60C above the temperature of the. contiguous container by thermal conduction through metal to said contiguous structure.

6. The apparatus according to claim 1 wherein said X-rays sources are mounted exteriorly and their radiation projected interiorly into the container through radiation permeable regions of relatively small area provided in the upper surface of the container.

7. The apparatus according to claim 6 wherein the resultant average density of free ions of both polarities within said gaseous space being at least two orders of magnitude higher than the average free ion density produced therein by naturally available ionizing processes.

8. A method of diminishing'to spark-free levels the intensity of unwanted electric fields within the gaseous spaces of a large container of flammable liquids and gases including .the steps of:'

a. providing at least one container housing a vaporizable liquid subject to accumulation of unwanted charge of one polarity and its vapor;

b. mounting a plurality of modular X-ray sources in a distributed array on the upper surface of the container, each source consisting ofa high voltage generator, an electron accelerating tube and a grounded anode X-ray target,

c. supplying electric power to the sources to maintain them in operation during extended periods when electrostatic hazards are presumed to exist, the X- rays emerging from the sources into the interior of the container as wide angle beams to produce free ions of both polarities throughout the gaseous spaces, those of said ions which have a polarity opposite to that of said unwanted charge moving toward said unwanted charge to neutralize the field effects of said unwanted charge.

9. The method according to claim 8 further including producing electrons within the X-ray source with energies less than 300 kiloelectron-volts.

10. The method according to claim 8 further including enclosing the X-ray sources in water-tight and gas-,

tight metallic enclosures.

11. The method according to claim 8 further includ ing maintaining the level of X-ray intensity one meter above the upper surface of the container below the internationally accepted safe limitfor human exposure.

' 12. The method according to claim 8 further inc'lud- 7 ing maintaining the target of the X-ray source at no more than C above the temperature of the contiguous container by thermal conduction through the metal to the contiguous structure.

13. The method according to claim 8 further including forming in the upper surface of the container radiation permeable regions of relatively small area, the

X-ray sources being mounted above theseregions ions of both polarities within the gaseous space at least two orders of magnitude higher than the average free ion density produced therein by naturally available ionizing processes.

15. In combination, a container adapted to be at least partially filled with a flammable liquid or with airborne solid particles, said liquid or said particles having a net unwanted electric charge of one polarity, said charge creating an electric field within the container which extends through the gaseous regions of said container, and an X-ray source to ionizing at least a portion of said gaseous region, said X-rays producing mobile free ions of both polarities, those of said ions which have a polarity opposite to that of said unwanted charge being attracted towards said charge to reduce the electric field within the gaseous region of said container to sparkfree levels.

16. The combination according to claim 15 wherein said X-ray source is supported outside said container and adapted to emit X-rays into said gaseous region,

the absorption of said X-rays in said gas producing ions.

uid-vapor interface of a flammable material, said ionization adapted to create mobile ions of both polarities which migrate within said container to neutralize the electric fields produced by the unwanted accumulation moderate amounts of X-ray energy to continuously ionof charges.

ize the gaseous region within a container housing a liq- 

1. Apparatus for diminishing to spark-free levels the intensity of unwanted electric fields within the gaseous spaces of large containers of flammable liquids and gases includinG: at least one container housing a vaporizable liquid subject to accumulation of unwanted charge of one polarity and its vapor, a plurality of modular X-ray sources, each source including a high voltage generator, an electron accelerating tube and a grounded anode Xray target, said plurality of sources being distributed in an array on the upper surface of said container, means for supplying electric power to said sources to maintain them in operation during extended periods when electrostatic hazards exist, said Xrays emerging from said sources into the interior of said container as wide angle beams to produce free ions of both polarities throughout said gaseous spaces, those of said ions which have a polarity opposite to that of said unwanted charge moving toward said unwanted charge to neutralize the field effects of said unwanted charge.
 2. The apparatus according to claim 1 wherein said X-rays are produced by electrons with energies generally less than 300 kiloelectron-volts.
 3. The apparatus according to claim 1 further including water-tight and gas-tight metallic enclosures for the X-ray sources.
 4. The apparatus according to claim 3 wherein the energy of said X-rays and the X-ray stopping power of the upper container surface and of the modular sources being such that the level of X-ray intensity 1 meter above this surface is always below the internationally accepted safe limit for human exposure.
 5. The apparatus according to claim 1 wherein the target of said X-ray source is maintained at no more than 60*C above the temperature of the contiguous container by thermal conduction through metal to said contiguous structure.
 6. The apparatus according to claim 1 wherein said X-rays sources are mounted exteriorly and their radiation projected interiorly into the container through radiation permeable regions of relatively small area provided in the upper surface of the container.
 7. The apparatus according to claim 6 wherein the resultant average density of free ions of both polarities within said gaseous space being at least two orders of magnitude higher than the average free ion density produced therein by naturally available ionizing processes.
 8. A method of diminishing to spark-free levels the intensity of unwanted electric fields within the gaseous spaces of a large container of flammable liquids and gases including the steps of: a. providing at least one container housing a vaporizable liquid subject to accumulation of unwanted charge of one polarity and its vapor; b. mounting a plurality of modular X-ray sources in a distributed array on the upper surface of the container, each source consisting of a high voltage generator, an electron accelerating tube and a grounded anode X-ray target, c. supplying electric power to the sources to maintain them in operation during extended periods when electrostatic hazards are presumed to exist, the X-rays emerging from the sources into the interior of the container as wide angle beams to produce free ions of both polarities throughout the gaseous spaces, those of said ions which have a polarity opposite to that of said unwanted charge moving toward said unwanted charge to neutralize the field effects of said unwanted charge.
 9. The method according to claim 8 further including producing electrons within the X-ray source with energies less than 300 kiloelectron-volts.
 10. The method according to claim 8 further including enclosing the X-ray sources in water-tight and gas-tight metallic enclosures.
 11. The method according to claim 8 further including maintaining the level of X-ray intensity one meter above the upper surface of the container below the internationally accepted safe limit for human exposure.
 12. The method according to claim 8 further including maintaining the target of the X-ray source at no more than 60*C above the temperature of the contiguous container by thermal conduction through the metal to the contiguous structure.
 13. The method according to claim 8 further including forming in the upper surface of the container radiation permeable regions of relatively small area, the X-ray sources being mounted above these regions whereby their radiation is projected interiorly into the container.
 14. The method according to claim 8 further including maintaining the resultant average density of free ions of both polarities within the gaseous space at least two orders of magnitude higher than the average free ion density produced therein by naturally available ionizing processes.
 15. In combination, a container adapted to be at least partially filled with a flammable liquid or with airborne solid particles, said liquid or said particles having a net unwanted electric charge of one polarity, said charge creating an electric field within the container which extends through the gaseous regions of said container, and an X-ray source fo ionizing at least a portion of said gaseous region, said X-rays producing mobile free ions of both polarities, those of said ions which have a polarity opposite to that of said unwanted charge being attracted towards said charge to reduce the electric field within the gaseous region of said container to spark-free levels.
 16. The combination according to claim 15 wherein said X-ray source is supported outside said container and adapted to emit X-rays into said gaseous region, the absorption of said X-rays in said gas producing ions.
 17. The method of preventing combustion of flammable material by electrostatic sparking caused by unwanted accumulation of charges including the steps of providing an X-ray source, and continuously delivering moderate amounts of X-ray energy to continuously ionize the gaseous region within a container housing a liquid-vapor interface of a flammable material, said ionization adapted to create mobile ions of both polarities which migrate within said container to neutralize the electric fields produced by the unwanted accumulation of charges. 